Introduction to Computer Networks and Network Models

Data Communication Fundamentals

  • Definition: Data communication is the exchange of data between two devices via some form of transmission medium.
  • Required Components: For data communication to occur, the communicating devices must be part of a communication system made up of a combination of hardware and software.
  • Characteristics of Effectiveness: The effectiveness of a data communication system depends on four fundamental characteristics:
    • Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.
    • Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.
    • Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. Timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay.
    • Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets.

Components of a Data Communication System

  • Message: The information to be communicated. Common formats include text, audio, and video.
  • Sender: The device that sends the data message.
  • Receiver: The device that receives the message.
  • Transmission Medium: The physical path by which a message travels from sender to receiver.
  • Protocol: A set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

Data Communication Modes

  • Simplex: Communication is unidirectional. Only one of the two devices on a link can transmit; the other can only receive (e.g., a mainframe to a monitor).
  • Half-Duplex: Each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive (e.g., walkie-talkies).
  • Full-Duplex (Duplex): Both stations can transmit and receive simultaneously. Signals going in one direction share the capacity of the link with signals going in the other direction.

Network Criteria and Performance

  • Network Definition: A network is a set of devices (nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending or receiving data.
  • Distributed Processing: Most networks use distributed processing, where a task is divided among multiple computers rather than one single large machine being responsible for all aspects of a process.
  • Performance Metrics:
    • Transit Time: The amount of time required for a message to travel from one device to another.
    • Response Time: The elapsed time between an enquiry and a response.
    • Key Metrics: Performance is evaluated by two networking metrics: throughput and delay.
    • Factors: Performance depends on the number of users, type of transmission medium, capabilities of connected hardware, and efficiency of software.
  • Reliability: Measured by the frequency of failure, the time it takes for a link to recover from a failure, and the network's robustness in a catastrophe.
  • Security:
    • Protecting data from unauthorized access.
    • Protecting data from damage and modification.
    • Implementing policies and procedures for recovery from breaches.
    • Security Breach: Any incident resulting in unauthorized access to data, applications, or devices, leading to information loss.

Types of Connections and Topologies

  • Connection Types:
    • Point-to-Point: Provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices (e.g., a television remote control and the TV control system).
    • Multipoint (Multidrop): More than two specific devices share a single link. Capacity is shared either spatially (simultaneous use) or temporally (taking turns).
  • Mesh Topology:
    • Every device has a dedicated point-to-point link to every other device.
    • Number of physical links in a fully connected mesh with nn nodes: n(n−1)2\frac{n(n-1)}{2}.
    • Advantages: Eliminates traffic problems (dedicated links), robust (one link failure doesn't affect the system), high security/privacy, and easy fault identification.
    • Disadvantages: Difficult installation/reconnection, bulk wiring issues, and expensive hardware for connecting every link.
  • Star Topology:
    • Each device has a dedicated point-to-point link only to a central controller, usually called a hub.
    • Advantages: Less expensive than mesh, easy to install/reconfigure (only one link per device), robust (single link failure only affects that link), and easy fault identification via the hub.
    • Disadvantages: Dependency of the whole topology on the central hub; requires more cabling than bus or ring.
  • Bus Topology:
    • A multipoint connection where one long cable (backbone) links all devices.
    • Drop Lines: Connections running between the device and the main cable.
    • Taps: Connectors that puncture the cable sheathing to create contact with the metallic core.
    • Advantages: Ease of installation and uses less cabling.
    • Disadvantages: Difficult reconnection and fault isolation; signal reflection at taps causes degradation; a fault in the main bus cable stops all transmission.
  • Ring Topology:
    • Each device has a dedicated point-to-point connection with only the two devices on either side of it.
    • Each device incorporates a repeater that regenerates bits.
    • Advantages: Easy to install/reconfigure (changing two connections for additions/deletions) and simplified fault isolation (alarm issued if no signal is received).
    • Disadvantages: Unidirectional traffic; a break in the ring can disable the entire network.
  • Hybrid Topology: A star backbone connecting multiple sub-networks (e.g., a star backbone with three bus networks).

Categories of Networks

  • Local Area Network (LAN):
    • Privately owned, linking devices in a single building, office, or campus.
    • Size limited to a few kilometers.
    • Designed for resource sharing (hardware, software, or data).
    • Common topologies: bus, ring, and star.
  • Wide Area Network (WAN):
    • Long-distance transmission over large geographic areas (countries, continents, or the world).
    • Switched WAN: Connects multiple end nodes through a common WAN network.
    • Point-to-Point WAN: A line leased from a provider connecting a computer or small LAN to an ISP.
  • Metropolitan Area Network (MAN):
    • Sized between LAN and WAN, covering an area inside a town or city.
    • Designed for customers needing high-speed connectivity (e.g., cable TV network).
  • Internetwork (Internet): Formed when two or more networks are connected. The Internet is a worldwide, public, autonomous facility accessed by hundreds of millions of people.

Hierarchical Organization of the Internet

  • History: Originated in the mid-1960s. ARPANET (Advanced Research Projects Agency Network) became a reality in 1969. Vint Cerf is considered the father of the Internet.
  • Structure:
    • International ISP: At the top of the hierarchy, connecting nations.
    • National ISP: Backbone networks maintained by specialized companies; connected via Network Access Points (NAPs).
    • Regional ISP: Smaller ISPs connected to one or more national ISPs.
    • Local ISP: Provides direct service to end users.

Intranet vs. Internet

  • Intranet: A secure, private communication network within a company. It is a restricted version of the internet that does not allow outside access.
  • Comparison Table:
    • Size: Internet is global/unlimited; Intranet is private/small.
    • Purpose: Internet is for global communication; Intranet is for corporate data sharing.
    • Regulation: Internet has no single authority; Intranet is regulated by the owning company.
    • Security: Internet depends on the user; Intranet is secured by firewalls.
    • Access: Internet is unrestricted/anonymous; Intranet is authorized users only.

Network Models: OSI and TCP/IP

  • Layered Model: Each layer represents specific functionality and protocols. Higher layers use the services of lower layers. Adjacent layers communicate via an interface.
  • Open Systems Interconnection (OSI) Model:
    • Developed by the ISO (International Standards Organization).
    • Consists of seven layers:
      1. Physical Layer: Coordinates functions to carry bit streams over a medium.
      2. Data Link Layer: Network support layer.
      3. Network Layer: Network support layer.
      4. Transport Layer: Links user support and network support layers; ensures end-to-end delivery.
      5. Session Layer: User support layer.
      6. Presentation Layer: User support layer.
      7. Application Layer: User support layer.
    • Encapsulation: Level N−1N-1 encapsulates the packet from Level NN without knowing its composition.
  • TCP/IP Model:
    • Developed by the Department of Defense (DoD) in the 1960s.
    • Consists of four layers:
      1. Network Access Layer: Combines the Physical and Data Link layers.
      2. Internet Layer: Protocols for logical transmission (e.g., IP).
      3. Transport Layer: Error-free end-to-end delivery (e.g., TCP, UDP).
      4. Application Layer: Highest layer, high-level protocols (e.g., HTTP, FTP, SMTP).

The Physical Layer and Signals

  • Responsibility: Movement of individual bits from one hop (node) to the next.
  • Concerns: Physical characteristics of interfaces, bit representation, data rate (bits/sbits/s), bit synchronization, line configuration, physical topology, and transmission mode.
  • Data and Signals:
    • Analog Data: Information that is continuous.
    • Digital Data: Information that has discrete states (11 and 00).
    • Analog Signal: Infinitely many levels of intensity over time.
    • Digital Signal: Limited number of defined values.
  • Analog-to-Digital Conversion (PCM):
    1. Sampling: Measuring the amplitude of the signal at equal intervals to create Pulse Amplitude Modulation (PAM) pulses.
    2. Quantization: Assigning integral values to the PAM pulses.
    3. Encoding: Converting values to binary equivalents (e.g., 88 bits; 77 for magnitude, 11 for sign).
  • Delta Modulation (DM): Finds the change from the previous sample. A positive change (delta) records a 11; a negative change records a 00. It creates a staircase-like signal.
  • Digital-to-Analog Conversion: Changing analog characteristics (amplitude, frequency, phase) to hold digital information.
    • Amplitude Shift Keying (ASK): Amplitude varies; frequency and phase remain constant.
    • Frequency Shift Keying (FSK): Frequency varies based on binary input.
    • Phase Shift Keying (PSK): Phase is altered (e.g., 2−PSK2-PSK uses 0∘0^{\circ} and 180∘180^{\circ}).

Transmission Media

  • Guided Media (Wired):
    • Twisted-Pair: Two insulated copper conductors twisted together. Twisting cancels out noise.
      • UTP: Unshielded Twisted-Pair.
      • STP: Shielded Twisted-Pair (bulkier/expensive, protects against noise).
    • Coaxial Cable: Central core enclosed in insulation and a metallic shield. Categorized by Radio Government (RG) ratings.
    • Fiber-Optic: Transmits light through glass or plastic cores.
      • Multimode Step-Index: Constant core density; light reflects at the cladding interface.
      • Multimode Graded-Index: Varying density (highest at center) to reduce signal distortion.
      • Single-Mode: Very small diameter; propagation is almost horizontal.
      • Advantages: High bandwidth, less attenuation (50 km50\,km without repeaters vs 5 km5\,km for copper), EM interference immunity.
  • Unguided Media (Wireless):
    • Ground Propagation: Waves follow Earth's curvature (below 2 MHz2\,MHz; e.g., AM radio).
    • Sky Propagation: Waves bounce off the ionosphere (2 MHz2\,MHz to 30 MHz30\,MHz; e.g., shortwave).
    • Line-of-Sight: Signals travel in straight lines (above 30 MHz30\,MHz; e.g., VHF, UHF, Microwaves).
    • Waves:
      • Radio Waves: 3 kHz3\,kHz to 1 GHz1\,GHz; omnidirectional; penetrate walls.
      • Microwaves: 1 GHz1\,GHz to 300 GHz300\,GHz; unidirectional; line-of-sight.
      • Infrared: 300 GHz300\,GHz to 400 THz400\,THz; short-range; closed areas.

Multiplexing Techniques

  • Purpose: Simultaneous transmission of multiple signals across a single data link to maximize bandwidth utilization.
  • Frequency-Division Multiplexing (FDM): Analog technique; each signal is assigned a unique frequency band. Signals are modulated onto different carrier frequencies (f1,f2,f3f_1, f_2, f_3).
  • Wavelength-Division Multiplexing (WDM): Analog technique for fiber-optics; combines different wavelengths (colors) of light.
  • Time-Division Multiplexing (TDM): Digital technique; each signal is assigned a specific time slot.
    • Synchronous TDM: Fixed time slots even if there is no data.
    • Asynchronous (Statistical) TDM: Time slots allocated dynamically based on need.